A fatigue warning device has been previously proposed by the present inventor, in which the fatigue warning system is combined with an ACC (adaptive cruise control) system. ACC systems are already in practical use and typically include as an environment sensor system a radar sensor installed in the front of the vehicle, which locates preceding vehicles, and a controller which automatically intervenes in the drive or brake system of the vehicle if the distance to the preceding vehicle drops below a predefined setpoint distance. If the distance to the preceding vehicle increases again, the controller causes the vehicle to accelerate, so that the distance is automatically regulated to the setpoint distance. The setpoint distance is usually defined by what is known as time gap, i.e., the time interval between the preceding vehicle and the host vehicle passing a fixed point on the roadway.
In some systems of this type, the driver has the option of setting the setpoint time gap within certain limits, for example, between one and two seconds.
In addition to ACC systems in which automatic distance regulation is performed, run-up alarm systems are also known in which only a warning signal is output if the time gap drops below a setpoint value, referred to as a warning distance, while the driver is left to control the vehicle. In the present specification, the term “warning distance” is used instead of the term “setpoint distance” even if actually not only a warning, but also an intervention in the drive or brake system, occurs.
In the device previously proposed by the present inventor, if the driver sensor system detects a driver fatigue condition, the vehicle is forcibly decelerated and finally braked to a standstill. The risk of an accident is thus minimized by the combination with the distance regulating system. If the roadway in front of the host vehicle is free, the vehicle decelerates with a moderate deceleration rate so as not to irritate or put at risk the traffic following behind. In contrast, if a preceding vehicle is detected by the environment sensor system, the host vehicle is decelerated more severely and more rapidly as a function of the measured distance, so as to prevent a collision with the rear of the preceding vehicle. The setpoint time gap set, however, is independent of the driver's fatigue condition.
Published international patent document WO 00/24309 describes a fatigue warning system in which a driving impairment is detected using a camera system which monitors the driver's eyelid movements. If driving impairment is detected, this warning system is designed not only to output a warning signal and turn on the hazard warning lights, but also to initiate controlled braking to safely brake the vehicle to a standstill.
In most known fatigue warning systems, only a warning signal is output to the driver when a fatigue condition is detected, for example, an optical or acoustic warning signal or also a haptic signal, for example, in the form of vibration of the steering wheel. However, these systems are not easily accepted by drivers, because the driver feels bothered by frequent warning signals most of which are subjectively perceived as unjustified.